In many places, we try to count pages using a 32 bit integer. That
implies if we are asked to create an object larger than 43bits, we will
subtly crash much later. Catch this on the boundary, and add a warning
to remind ourselves later on our exabyte systems.

Looks like it wouldn't detect storing unsigned int in a signed int but I
guess we don't care that much as long as this is local use only. Just
slightly relevant because of the int page_count situation we mention below.